Growth of solid layers on substrates which are kept under ion bombardment before and during deposition



Feb. 13, 1962 G. WEHNER 3,021,271

GROWTH OF SOLID LAYERS 0N SUBSTRATES WHICH ARE KEPT UNDER IONBOMBARDMENT BEFORE AND DURING DEPOSITION Filed April 27, 1959 300v.200v. o0

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ATTORNEY United States Patent GROWTH OF SOLID LAYERS ON SUBSTRATES WHICHARE KEPT UNDER ION BOMBARD- MENT BEFORE AND DURING DEPOSITION GottfriedWehner, Minneapolis, Minn., assignor to General Mills, Inc., acorporation of Delaware Filed Apr. 27, 1959, Ser. No. 809,237 3 Claims.(Cl. 204-192) above mentioned methods the critical parameters are thesubstrate temperature and the corresponding rate of deposition. Thetemperature apparently-determines, among other things, the rateoftsiirface migration of the arriving atoms of the deposited materialand, as such, the activation energy in the form of heat necessary totransport the deposited atoms to the location of the lowest free energy,i.e. their right lattice positions. These attempts, except for certainspecial and simple cases involving layers of preferred orientation orvery thin single crystal layers,

' have generally been unsuccessful due to the difiiculty of removal ofoxide or other chemically adsorbed layers from the substrate surface,which interfacial obstruction serves to block the growth andcontinuation of the substrate crystal structure.

This invention may be briefly described as a method of effecting thegrowth of solid layers on substrates which includes, in its broaderaspects, the step of maintaining the substrate surface under controlledcondition ionic bombardment before and during deposition. In moreparticularity, the herein described invention includes utilization ofionic bombardment of such energy and current density as to provide theactivation energy necessary to move the deposited atoms to their rightlattice positions and to effect sufficient sputtering of the substratesurface as to effectively remove contaminant surface layers therefromprior to deposition and to materially contribute to the maintaining ofan atomically clean substrate surface during deposition.

Among the advantages of the herein described invention is a wideincrease in the number of permitted combinations of deposit andsubstrate materials determinable, at least in'part, by congruity ofcrystal configuration and similarity of lattice constants, the removalof foreign atoms, stacking faults and dislocations from the substratesurface and consequent permitted increase in the degree of bondperfection and the permitted utilization of polycrystalline materials asone of the solid components. The invention described herein, due atleast in part to the advantages set forth above, is possessed of markedutility in most, if not all, fields wherein metal, semi-conductor orinsulating layers are desirably atomically bonded to a substrate surfacesuch as in, for example, the fabrication of semi-conductors, thepreparation of surfaces for electrical contacts, for the preparation ofcorrosion resistant surfaces or for covering surfaces withheat-resistant material, such as tungsten.

The principal object of the invention is the provision of an improvedmethod for effecting the deposition of materials on substrates.

Another and further object of this invention is the proice vision of animproved method for efiecting the growth of solid layers on substrateswith an improved atomic bond therebetween and with a continuance ofsubstrate crystal orientation in the deposited material.

Other objects and advantages of the invention will be pointed out in thefollowing specification and claims and illustrated in the accompanyingdrawings which depict one type of apparatus suitable for practicalrealization of the advantages attendant practice of the herein disclosedmethods.

Referring to the drawings:

FIGURE 1 is a schematic representation of the essentials of one type ofapparatus by which practice of the herein disclosed invention may beeffected.

In its broad aspects the invention herein disclosed includes the step ofsubjecting a substrate surface to ionic bombardment to effect theremoval of contaminant material therefrom and to otherwise prepare thesame for the receipt of the material to be deposited prior to depositionthereof and the step of maintaining said substrate surface undercontrolled ionic bombardment during deposition so as to effect at saidsubstrate surface the ratio of the material deposited thereon to thematerial sputtered away therefrom and to maintain the overall rate ofdeposition below a predetermined critical value.

In more particularity, desirable growth conditions in the practice ofthe herein described invention may be conveniently achieved byimmersing, in face to face relationship, the substrate and the sourcefor material to be deposited as separate electrodes in a low pressurehigh density gas discharge plasma with low background impurity pressure,such as a DC. mercury vacuum arc plasma of the type created at about 1micron mercury gas pressure between a pool type cathode and an anode.With the material to be deposited and the substrate so disposed, bothare subjected to preliminary ionic bombardment to effect the removal ofcontaminant surface layers therefrom and otherwise prepare the substratesurface for receipt of the material to be deposited. After apredetermined cleaning period, the potentials on the material to bedeposited and the substrate are modified to effect selective andcontrolled condition ionic bombardment thereof so as to effect thearrival at the substrate surface of more of the material that is to bedeposited than is being sputtered away therefrom and to control theoverall rate of deposition so as to maintain the same below apredetermined value.

The above mentioned initial ionic bombardment effects a sputtering ofboth the surfaces of the substrate and the material to be deposited andin the removal of oxide or other chemically adsorbed contaminant layerstherefrom as well as possibly correcting possible stacking faults anddislocation from the substrate surface. Under the selective andcontrolled condition ionic bombardment sputtering of the surface of thematerial to be deposited continues as does the sputtering of thesubstrate surface. However, if the operational conditions are controlledso that a greater quantity of the material to be deposited arrives atthe substrate surface than is being continually sputtered away therefromand if the overall rate of deposition upon the substrate surface ismaintained below certain critical values, which apparently vary with thesubstances involved, growth is effected characterized by a closeapproach to, if not actual attainment of, a true atomic bond and acontinuance of substrate crystal orientation in the deposited material.If the critical rate of deposition is exceeded, the deposit will notusually grow as a single crystal but will usually show only a preferredorientation of very small crystallites. As such the deposition rate isprobably one of the major determinants of the perfectnessof thedeposited crystal.

It is believed that under the deposition conditions de- 3 scribed hereinthat the bombarding ions provide the necessary activation energy for themovement of the deposited atoms into their right lattice positions.

By way of general example, growth conditions of the type hereindescribed may conveniently be obtained through utilization of apparatusof general character schematically illustrated in FIGURE 1 and as alsogenerally shown and described by the inventor hereof in conjunction withcertain published studies at pages 690-704 in vol. 102, No. 3 of ThePhysical Review and, for example, a German article cited thereinbeginning on page 501 of Annalen Der Physik 5 Folge Bd 71 Heft 7 u. 8(1942). As depicted in FIGURE 1, suitable apparatus may include ademountable tube having upper and lower sections 12 and 14,respectively, with a properly designed fine mesh graphite grid 16 whichseparates the tube anode space from the cathode space thereof. The upperand the lower sections of the tube are suitably sealed together byrubber gaskets 18. The graphite grid 16, which may be about 0.4 mm.thick having about thirty-six holes per cm. of 1.3 mm. diameter asexplained in the above identified publication, permits a considerableincrease in plasma density within the anode space in the upper tubesection 12 without utilization of undesirably high discharge currentsand also permits an appreciable, yet simple, control of the velocity ofaccelerated beam electrons by variations on the grid potential.

The lower tube section 14 preferably contains a suitable exhaust pumpconduit 20, a mercury pool cathode 22 having a cathode spot anchor 24and an igniter 26. The upper portion of the lower tube section 14contains an axially disposed auxiliary anode 28.

The upper tube section 12 contains an anode 30 preferably disposedoutside of the path of the beam electrons and an associated repeller 32.The anode 30 and repeller 32 are preferably positioned so as to efiect areflection of the beam electrons back into the plasma and therebyproduce a further increase in plasma density within the anode space ofthe tube. If desired, a still further increase in plasma density may bereadily effected by application of suitable magnetic fields. Immersedwithin the area of high plasma density within the anode space is thesubstrate or seed crystal 34. Facing the substrate surface and disposedin closely spaced relationship therewith is the source of the materialto be deposited, conveniently termed the depositor 36.

Circuitry for the operation of the low pressure plasma discharge liquidmercury pool tube of FIGURE l is well known in the art. For example, acircuit such as illustrated on page 507 of the previously noted articlebeginning on page 501 of Annalen Der Physik 5 Folge Bd 41 Heft 7 u. 8(1942) would be adequate to establish the necessary plasma density inthe upper tube section 12. The biases on the substrate 34 and thedepositor 36 mentioned below can be obtained, as is well known in theart, by the use of a battery with appropriate dropping resistors andswitches.

By way of specific example of the practice of the herein disclosedmethod with employment of the above described tube, germanium crystal,prepared to have a polished and chemically etched plane (100) surface,was immersed in the region of high plasma density as the substrate. Alsoimmersed in the region of high plasma density and spaced about 2 to 3cm. away from the above described substrate, was the source of materialto be deposited, specifically a piece of very pure polycrystallinegermanium. The substrate and depositor surfaces were then subjected tocleaning by ionic bombardment for about minutes under conditionsapproximating 300 volts ion energy and 5 ma./cm. ion current density.After the ionic bombardment cleaning period and without interruption thepotential of the germanium crystal substrate was changed to minus 100volts with respect to the anode, and that of the depositor to minus 200volts with respect to the anode. Under these conditions and at roughly 5ma./cm. ion current density approximately eight germanium monolayerswere sputtered from the depositor per second. The sputtering rate fromthe germanium substrate was, at about e.v. ion energy, approximately onemonolayer per second. Under such conditions the deposited germaniumgrows on the germanium substrate as a single crystal with a continuanceof substrate crystal orientation at the rate of roughly 0.5;1. per hour.

If the overall rate of growth is too rapid, as for example, as would beobtained by application of minus 300 volts to the depositor (withrespect to the anode) instead of the minus 200 volts as set forth in theabove example, the deposited germanium does not grow as a single crystalbut shows only a preferred orientation of very small crystallites.

In order to efiect proper growth on the substrate it is necessary toprevent surface contamination thereof by external contaminants such asthe gas contained within the tube. In the above described examplecontamination of the substrate surface by condensation of mercury atomsthereon may be eifectively prevented by maintaining the substrate at atemperature of at least 250 C. In the described example and under theconditions of operation described, the target automatically assumed atemperature of about 300 C. under the intense ion bombardment which waswell above any temperature at which undesired contamination could beeffected by condensation of mercury atoms.

Another example illustrative of the practice of the herein disclosedinvention with employment of the above described tube included theutilization of a piece of optical grade silicon as the depositordisposed about 2 centimeters away from the substrate in the region ofthe high plasma density. The particular substrate again was a germaniumcrystal having a polished and chemically etched plane (100) surface.With the depositor and substrate so positioned, they were both subjectedto cleaning by ionic bombardment for about 15 minutes under conditionsapproximating 300 volts ion energy and 5 ma./cm. ion current density.After the cleaning period, and without interruption, the potential ofthe germanium crystal substrate was changed to minus 100 volts withrespect to the anode and that of the silicon depositor, which has alower sputtering rate, to minus 500 volts with respect to the anode.Under these conditions and at roughly 5 ma./cm. ion current densityapproximately six silicon monolayers are sputtered from the depositorper second with the sputtering rate from the germanium crystal substrateagain being roughly 1 monolayer per second. Under such conditions thedeposited silicon grows on the germanium substrate at a rate of about0.3,u per hour. Here again, if the overall rate growth is too rapid, thedeposit does not grow as a single crystal but shows only a preferredorientation of very small crystallites.

The above examples are intended only to be illustrative of the practiceof the method herein disclosed. The values set forth may not be, andprobably are not, the optimal growth values and will change withmaterials employed. geometric arrangements and other variables thatwould be attendant any given operation. The overall considerations,however. which apparently determine the actual values to be desirablyemployed include, at least that more depositor material arrives at thesubstrate surface than is being sputtered away therefrom and that theoverall rate of deposition of the depositor material on the substratesurface remain below a certain value, since this latter is probably amajor factor or determinant of the perfectness of the deposited crystal.

Although the above described examples relate to operations effected bymeans of the illustrated apparatus wherein release of the depositormaterial was eifected by sputtering under ion bombardment, the inventionis not so limited. For example, the depositor material could eon-ally aswell have been released by evaporation. In

such case, the depositor could be made an anode and be heated up to thenecessary temperatures by electron bombardment or by inclusion of aseparate heating element as is well known in the art. As will also beapparent to those skilled in the art, the illustrated positionalarrangement between the depositor and substrate can readily be modifiedwithout departure from the principles of the herein described inventionas for example, having the depositor concentrically, or evenspherically, enclose the substrate so as to minimize loss of depositormaterial. Moreover, the discharge plasma may as well be created by othermeans than a DC. discharge, as for example by highfrequency ionizationof a low pressure gas. Likewise practice of the invention is notconfined to a mercury plasma, other noble gases like He, Ne, A, Kr andXe may be used just as well for the ion bombardment, In addition, theherein described method is not intended to be limited to single crystalgrowth but may well find utility in the more general covering ofpolycrystalline materials with other materials with improved bondtherebetween.

Having thus described my invention, what I claim is:

1. In the growing of solid layers on substrates the steps of immersingthe substrate and the material to be deposited thereon as separateelectrodes in a low pressure supported gas discharge plasma of highdensity established between two other electrodes, subjecting saidsubstrate and said material to ionic bombardment to effect the removalof contaminant material therefrom, then lowering the potential of saidsubstrate and said material so the potential of said material is morenegative than the potential of said substrate, and subsequentlysubjecting said substrate and said material to selective ionicbombardment such that atoms of said material are sputtered from saidmaterial to said substrate and from said substrate in a manner such thatthe rate of said atoms arriving at said substrate is higher than therate of said atoms leaving said substrate.

2. In the growing of solid layers on substrates the steps of immersingthe substrate and the surface of the material to be deposited asseparate electrodes facing each other in a low pressure supported gasdischarge plasma of high density established between two otherelectrodes, subjecting said substrate surface and said material surfaceto ionic bombardment with positive rare gas ions to effect the removalof contaminant material therefrom, then and without interruptionlowering the potential of said substrata and said material surface sothe potential of said material surface is more negative than thepotential of said substrate, then subjecting said substrate and saidmaterial surface to an ion current density such that atoms of saidmaterial surface are sputtered from said surface to said substrate, saidcurrent density being such that the rate of atoms arriving at saidsubstrate surface is higher than the rate of atoms sputtered awaytherefrom.

3. In the growing of solid layers on substrates the steps of immersingthe substrate and the surface of the material to be deposited asseparate electrodes facing each other in a low pressure supported gasdischarge plasma of high density established between two otherelectrodes, subjecting said substrate surface and said material surfaceto ionic bombardment with Hg-ions to effect the removal of contaminantmaterial therefrom, then and without interruption lowering the potentialof said substrate and said material surface so the potential of saidmaterial surface is more negative than the potential of said substrate,then subjecting said substrate and said material surface to an ioncurrent density such that atoms of said material surface are sputteredfrom said surface to said substrate, said current density being suchthat the rate of atoms arriving at said substrate surface is higher thanthe rate of atoms sputtered away therefrom.

References Cited in the file of this patent UNITED STATES PATENTS2,242,042 Paetow May 13, 1941 2,394,930 McRae Feb. 12, 1946 2,677,071Carne Apr. 27, 1954 2,754,259 Robinson et a1 July 10, 1956 2,843,542Callahan July 15, 1958

1. IN THE GROWING OF SOLID LAYERS ON SUBSTRATES THE STEPS OF IMMERSINGTHE SUBSTRATE AND THE MATERIAL TO BE DEPOSITED THEREON AS SEPARATEELECTRODES IN A LOW PRESSURE SUPPORTED GAS DISCHARGE PLASMA OF HIGHDENSITY ESTABLISHED BETWEEN TWO OTHER ELECTRODES, SUBJECTING SAIDSUBSTRATE AND SAID MATERIAL TO IONIC BOMBARDMENT TO EFFECT THE REMOVALOF CONTAMINANT MATERIAL THEREFROM, THEN LOWERING THE POTENTIAL OF SAIDSUBSTRATE AND SAID MATERIAL SO THE POTENITAL OF SAID MATERIAL IS MORENEGATIVE THAN THE POTENTIAL OF SAID SUBSTRATE, AND SUBSEQUENTLYSUBJECTING SAID SUBSTRAT AND SAID MATERIAL TO SELCTIVE IONIC BOMBARDMENTSUCH THAT ATOMS OF SAID MATERIAL ARE SPUTTERED FROM SAID MATERIAL TOSAID SUBSTRATE AND FROM SAID SUBSTRATE IN A MANNER SUCH THAT THE RATE OFSAID ATOMS ARRIVING AT SAID SUBSTRATE IS HIGHER THAN THE RATE OF SAIDATOMS LEAVING SAID SUBSTRATE.